docs: def add_job_to_db(self, key, job):
          """Add job info to the database."""
          job_msg = self.registry.deep_encode(job)
          prov = prov_key(job_msg)
  
          def set_link(duplicate_id):
              self.cur.execute(
                  'update "jobs" set "link" = ?, "status" = ? where "id" = ?',
                  (duplicate_id, Status.DUPLICATE, key))
  
          with self.lock:
              self.cur.execute(
                  'select * from "jobs" where "prov" = ? '
                  'and (("result" is not null) or '
                  '("session" = ? and "link" is null))',
                  (prov, self.session))
              rec = self.cur.fetchone()
              rec = JobEntry(*rec) if rec is not None else None
  
              self.cur.execute(
                  'update "jobs" set "prov" = ?, "version" = ?, "function" = ?, '
                  '"arguments" = ?, "status" = ? where "id" = ?',
                  (prov, job_msg['data']['hints'].get('version'),
                   json.dumps(job_msg['data']['function']),
                   json.dumps(job_msg['data']['arguments']),
                   Status.WAITING,
                   key))
  
              if not rec:
                  # no duplicate found, go on
                  return 'initialized', None
  
              set_link(rec.id)
  
              if rec.result is not None and rec.status == Status.WORKFLOW:
                  # the found duplicate returned a workflow
                  if rec.link is not None:
                      # link is set, so result is fully realized
                      self.cur.execute(
                          'select * from "jobs" where "id" = ?',
                          (rec.link,))
                      rec = self.cur.fetchone()
                      assert rec is not None, "database integrity violation"
                      rec = JobEntry(*rec)
  
                  else:
                      # link is not set, the result is still waited upon
                      assert rec.session == self.session, \
                          "database integrity violation"
                      self.attached[rec.id].append(key)
                      return 'attached', None
  
              if rec.result is not None:
                  # result is found! return it
                  result_value = self.registry.from_json(rec.result, deref=True)
                  result = ResultMessage(
                      key, 'retrieved', result_value, None)
                  return 'retrieved', result
  
              if rec.session == self.session:
                  # still waiting for result, attach
                  self.attached[rec.id].append(key)
                  return 'attached', None